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SiOx films (80 nm thick) were vacuum deposited onto a 25-μm-thick polyethylene terephthalate (PET) substrate under various conditions. The effects of chamber pressure, evaporating temperature, and chamber gas composition on the time evolution of the oxygen transmission rate (OTR) were investigated together with changes in the films composition. It was found that the OTR decreases over time when the films are left in air, the rate of the decrease being greater at higher pressures during the formation of the films. However, if the pressure is too high, the OTR immediately reaches a high value after evaporation and does not decrease over time, not even if the films are left in air for a week. It is possible to obtain acceptable low OTR values by employing a moderate operating pressure, a suitable evaporating temperature, and an aging treatment after deposition without using a high-capacity vacuum system and without heat damage of the plastic film substrate.